1. Field of the Invention
The present invention relates to a safety apparatus for a perforating system.
2. Description of the Prior Art
Safety is a substantial issue in connection with the providing of perforating services and procedures and is of the utmost concern on a tubing conveyed perforating (“TCP”) job when the firing head is first attached to the gun string or when a firing head is removed from a non-detonated gun string which has been retrieved from its downhole position. The attachment or removal of the firing head has typically been done on the rig floor when personnel are near the equipment, and if detonation occurs at this time, severe property damage and bodily harm, including death, may result.
One safety technique which has been employed is to install a predetermined length of pipe with no shape charges, e.g., 10 feet, on top of a gun string prior to installing the firing head. By following this procedure, the gun string is below the rig floor when the firing head is being installed. If the guns were to detonate when the firing head is installed, the harm to human life would be somewhat protected inasmuch as personnel are not directly in the line of fire of the guns. Another safety technique has been to require that two parameters, e.g., mechanical action and pressure, be satisfied in order to detonate the firing head. At the surface where the firing head is installed, there is typically insufficient pressure to satisfy the pressure requirements, and the firing heads can be considered safe while at the surface.
Other safety systems employ techniques where the firing head is mechanically blocked until the system is downhole near the zone to be perforated. These systems are known as “downhole arming” systems and can include eutectic material which is solid at low temperatures and melts at slightly higher temperatures. These eutectic materials function to block a firing pin from impacting a detonator at the surface. When the system is run downhole, however, the eutectic material melts and the firing pin has a clear path to strike the detonator. A drawback to this type of system is that, once the eutectic material melts, it flows away from its original blocking cavity. Thus, when a non-detonated gun string is retrieved from the well, the path of the firing pin to the detonator would no longer be blocked.
Other safety methods may include a utilization of techniques to interrupt the ballistic train from the firing head to the gun string. One such device called a “hammer stop” also utilizes eutectic material as described above. The eutectic material is physically positioned to block the path of the firing pin from impacting a percussion detonator. As the system is run into the well, the downhole temperature increases and the eutectic material melts. The physical barrier between the firing pin and the detonator is thus removed.
Another ballistic interruption-type safety method involves the use of a radial blocking pin. The pin is positioned between a firing pin and a percussion detonator, and the pin is held in the blocked position by a spring. The pin has O-ring seals and seals against an atmospheric chamber. As the system is run into the hole, well pressure causes the pin to move against the atmospheric chamber. When the pin is moved to its final position, a hole in the pin allows the firing pin to have a clear path to the detonator.